Organic light-emitting diode (OLED) devices, also referred to as organic electroluminescent (EL) devices, have numerous well-known advantages over other flat-panel display devices currently in the market place. Among the potential advantages is brightness of light emission, relatively wide viewing angle, reduced device thickness, and reduced electrical power consumption compared to, for example, liquid crystal displays (LCDs) using backlighting.
Applications of OLED devices include active-matrix image displays, passive-matrix image displays, and area-lighting devices such as, for example, selective desktop lighting. Irrespective of the particular OLED device configuration tailored to these broad fields of applications, all OLEDs function on the same general principles. An organic electroluminescent (EL) medium structure is sandwiched between two electrodes. At least one of the electrodes is at least partially light transmissive. These electrodes are commonly referred to as an anode and a cathode in analogy to the terminals of a conventional diode. When an electrical potential is applied between the electrodes so that the anode is connected to the positive terminal of a voltage source and the cathode is connected to the negative terminal, the OLED is said to be forward biased. Positive charge carriers (holes) are injected from the anode into the EL medium structure, and negative charge carriers (electrons) are injected from the cathode. Such charge carrier injection causes current flow from the electrodes through the EL medium structure. Recombination of holes and electrons within a zone of the EL medium structure results in emission of light from this zone that is, appropriately, called the light-emitting zone or interface. The organic EL medium structure can be formed of a stack of sublayers that can include small molecule layers or polymer layers. Such organic layers and sublayers are well known and understood by those skilled in the OLED art.
Full-color OLED devices may employ a variety of organic materials to emit different colors of light. In such arrangement, the OLED device is patterned with different sets of organic materials, each set of organic materials associated with a particular color of light emitted. Each pixel in an active-matrix full-color OLED device typically employs each set of organic materials, for example to form a red, green, and blue sub-pixel element. The patterning is typically done by evaporating layers of organic materials through a mask. In an alternative arrangement, a single set of organic materials emitting broadband light may be deposited in continuous layers on the substrate, and a full-color OLED device formed by employing an array of differently colored filters in combination with the broadband emitted light.
The emitted light is directed towards an observer, or towards an object to be illuminated, through the light transmissive electrode. If the light transmissive electrode is between the substrate and the light emissive elements of the OLED device, the device is called a bottom-emitting OLED device. Conversely, if the light transmissive electrode is not between the substrate and the light emissive elements, the device is referred to as a top-emitting OLED device. The present invention may be directed to either a top-emitting or bottom-emitting OLED device. Reflective electrodes may be made of relatively thick and electrically conductive metal compositions that can be optically opaque, for example employing metals such as silver or aluminum or alloys employing such metals. Typical prior-art materials proposed for transparent electrodes include indium tin oxide (ITO) and very thin layers of metal, for example silver, aluminum, or magnesium or metal alloys including silver, aluminum, or magnesium.
Referring to FIG. 2, a top-emitting OLED device as suggested by the prior art is illustrated having a substrate 10 (either reflective, transparent, or opaque). Over the substrate 10, a semiconducting layer is formed providing thin-film electronic components 30 for driving an OLED. An interlayer insulating and planarizing layer 32 is formed over the thin-film electronic components 30 and a patterned reflective electrode 12 defining OLED light-emissive elements is formed over the insulating layer 32. An inter-pixel insulating film 34 separates the elements of the patterned reflective electrode 12. One or more first layers 14 of organic material, one of which emits light, are formed over the patterned reflective electrode 12. A transparent second electrode 16 is formed over the one or more first layers 14 of organic material. A gap separates the transparent second electrode 16 from an encapsulating cover 20. The encapsulating cover 20 is transparent and may be coated directly over the transparent electrode so that no gap exists. In some prior-art embodiments, the first electrode 12 may instead be at least partially transparent and/or light absorbing.
As shown in FIG. 2, the organic layers 14 need not be patterned, and broadband light, for example white light, can be emitted from the organic layers 14, through the color filters 24R, 24G, and 24B to form a color display. The color filters 24R, 24G, and 24B, transmit only a desired color of light, for example red, green, and blue corresponding, for example to the desired colors in a color display. The colored light 50, 51, 52 then passes through the cover 20 and is emitted from the device. The active-matrix thin-film electronic components 30, for example bus wires, capacitors, thin-film transistors and the like may be employed to provide power to the OLED elements. Alternatively, as is known in the prior art, a passive-matrix control scheme with separate power lines for each row or column of OLED elements may be employed to provide power to the OLED elements. Alternatively, the organic layers 14 may be patterned so that differently colored light is emitted from different locations on the device.
OLED materials have different light emission properties, and it is known that some colors of light are emitted more efficiently than others, in particular white emitters are known that have an efficiency higher than that of both blue and red emitters. Hence, it has been proposed to employ OLED pixels having four sub-elements: red, green, blue, and white (RGBW). Because most images have a large luminance component, such four-element displays can be more efficient than conventional three-element displays. Such designs are described, for example, in U.S. Pat. No. 6,919,681 and US2004/0113875.
Referring to FIG. 3, a top-emitting OLED device having four sub-elements: red, green, blue, and white (RGBW) is illustrated. In this arrangement, the organic layers 14 emit broadband, for example substantially white, light that passes through the color filters 24R, 24G, and 24B. The light emitted from the fourth sub-pixel is typically unfiltered. However, if the color of the white light does not match the desired white-point of the device, white point adjusting filters or white color filters are disclosed in U.S. Pat. No. 6,919,681 and US2004/0113875 for possible use with the broad-band white light emitting elements for modifying the color of emitted light if desired. The designs of both FIGS. 2 and 3 may also be applied to a bottom-emitting structure. In particular, a bottom-emitting version of FIG. 3 has been constructed by Applicant.
In many OLED display applications, it is necessary to employ an OLED display outdoors in high brightness ambient conditions since flat-panel display devices are widely used in conjunction with communication devices and in particular with portable devices. These displays are often used outdoors or in public areas with significant ambient illumination. In these locations, the contrast of the display is of great concern. In particular, OLED display devices suffer from problems with contrast since the back electrode of the devices is typically highly reflective.
It is known to employ circular polarizers with flat-panel OLED displays to reduce the reflection of ambient light on the front of the flat-panel displays and thereby improve the contrast of the display. Circular polarizers are known to improve contrast in light emitting displays, for example, as disclosed in US2004/0189196. Circular polarizers comprise a linear polarizer and a quarter-wave plate. Light (such as emitted light) that passes through the quarter wave plate and polarizer once is polarized. Light (such as ambient light) that passes first through the polarizer, then the quarter-wave plate and is subsequently reflected back through the quarter-wave plate and the filter is largely absorbed. However, such circular polarizers are expensive.
It is also known to employ neutral density filters to improve the contrast of an emissive display device. For example, a neutral density filter may be applied to all of the pixel sub-elements in common. Alternatively, an actively-controlled filter may be employed. US20050122053 A1 entitled “Organic electroluminescent display” describes an organic electroluminescent display having a transparent display panel, a reflective sheet and a brightness regulating film between the substrate and the reflective sheet. The regulating film is able to adjust the light intensity transmitting of the OLED display in response the level of ambient illumination by using a photochromic material. In these designs, light emitted by each of the OLED elements will be absorbed by some amount determined by the density of the neutral density filter. Ambient light that is reflected and re-emitted through the neutral density filter will be absorbed twice in an amount equal to the square of the absorption amount, thereby improving the contrast of the device. However, as taught in the prior art such a solution also reduces the brightness of the color emitters and the efficiency of the display is reduced.
There is a need therefore for an improved organic light-emitting diode device structure that increases the light output and improves contrast.